One of the somewhat chilling things one learns if one ever looks into the matter is that the human race is an infanticidal species; we kill our young quite regularly. Infanticide is almost universal, and some people estimate that in most cultures anywhere from ten to forty percent of otherwise surviving infants are deliberately killed. One hears, of course, of Carthage -- both the Jews and the Romans attest to the Carthaginian practice of sacrificing children to some sort of deity (Moloch in the Bible, Baal according to most of the Romans). Most cultures considered ritual sacrifice of children to be barbaric, but that's not the same as frowning at infanticide itself. The Greeks and Romans (and many other cultures) practiced exposure of infants, a practice that keeps coming up if you read their myths. We know from Aristotle that in some places there were restrictions on this, but they seem to have been relatively slight. But the evidence for widespread killing of newborn infants is quite extensive, and it is almost a constant across cultures.
Almost. All of the major monotheistic religions (Judaism, Christianity, Islam, Sikhism) have attempted to stamp out the practice, and have usually had some success, although in all cases the progress was slow and the degree of success varied from culture to culture. Nonetheless, there is an argument that monotheism has saved more infants than anything else in history; part of the reason is that all the major monotheistic religions are big institution-builders and therefore they didn't just oppose infanticide but built institutions that partly reduced the temptation. Nonetheless monotheistic religion, while perhaps the most successful, is not the only thing that historically has opposed infanticide. The Qin and Han dynasties tried to make it illegal in China -- for population reasons, I think -- but they seem to have had considerable difficulty enforcing the laws. The religion of the ancient Egyptians also opposed infanticide, and in fact the ancient Greeks were often surprised at the sheer commitment of the Egyptians in saving infants from exposure -- people of Egyptian culture would save infants from hillsides and trash heaps and raise them as their own (although sometimes as household slaves). How the Egyptians managed to get so many of their own people on the same page is a bit of a mystery; the Egyptian view of children is not otherwise noticeably different from that of their neighbors. The Egyptians are almost unique. While we have much less certainty about them, the Etruscans are also said by some to have opposed infanticide, although we don't know for sure. Both of these seem to be religiously driven, but there seem also to have been cases of philosophical opposition to the practice; both Musonius Rufus and Epictetus are firmly opposed to it, although this doesn't seem to have been universal among Stoics. This might not be significant as a social matter, but on the other hand, if there was Stoic opposition, this might be one of the reasons for the fact that the late Roman Empire sees a slow rise in legal restrictions of the practice. But it's hard to say.
And, of course, this is all out-and-out infanticide. When one includes abortion, the practice of killing children is even more extensive. The monotheistic religions tend again to be major barriers, although Christianity stands out as having a history of much stronger and less qualified prohibitions against the practice than is usually the case; unequivocal opposition to abortion is found in some of the earliest Christian writings on morals that we have, like the Didache.
Beyond all this, you have individuals here and there who come to similar oppositions on philosophical grounds, but not much more. Left to themselves, human beings drift quickly toward abortion and, more slowly, but nonetheless definitely, toward infanticide. Only religion and reason usually ever stand in the way, and both have varying degrees of success in doing so, when they even do so.
